1. Field of the Invention
This invention is in the field of pressure filter systems in which a pressure differential in the liquid being filtered is maintained across a filter element and particularly to such systems in which the filter elements are hollow porous wall filter tubes whose filtering capability are significantly enhanced by forming on the surface of the porous walls of the filter tubes filter cake from finely divided filter material by causing the liquid portion of a slurry of the filter material and liquid to flow through the filter tubes. More particularly this invention is in the field of methods and apparatus for regenerating the filter cake of such filter systems so that the useful life of the filter material forming the filter cake is significantly extended.
2. Description of the Prior Art
Pressure filter systems for removing undesirable insoluble solids from a liquid such as water are well known in the art. Typically these systems have a pressure vessel whose interior is divided into a filter chamber into which the liquid to be filtered is introduced and a filtrate chamber into which the filtered liquid flows. The filtering elements are hollow filter tubes having porous walls which are mounted in the filter chamber with the interior of the filter tubes in communication with the filtrate chamber. The mounting means for the filter tubes divides or isolates the two chambers so that liquid can flow from the filter chamber to the filtrate chamber only after passing through a filter tube. The surfaces of the filter tubes have built up on them a layer of filter material, diatomaceous earth, to form a filter cake. The filter cake is produced by forming a slurry of diatomaceous earth with the liquid to be filtered, water for example, in a precoat tank and the slurry is then pumped into the filter chamber. As the liquid portion of the slurry flows through the porous walls of the filter tubes, the diatomaceous earth builds, or forms, the filter cake on the exterior cylindrical surfaces of the porous walls of the filter tubes. After the filter cake is formed the liquid to be filtered is pumped into the filter chamber and flows through the filter cake into the hollow portion of the filter tubes and through the tubes to the filtrate chamber. The filtrate then flows through an outlet pipe to where it is to be used or stored.
Undesirable elements in the fluid being filtered, impurities, are trapped or retained on the outer surfaces of the filter cake. As filtration continues the solids retained on the surface of the filter cake create a substantially impermeable crust. The flow rate of the liquid through the filter system is reduced and the pressure in the filter chamber increases. When the efficiency of the filter system decreases due to the resistance of the filter cake to the flow of fluid through it because of the impurities on the surface, the prior art teaches regenerating the filter cake by backflushing the filter system. In a backflushing operation, the liquid in the filter system is forced to flow in the opposite direction from normal through the porous walls of the filter tubes to remove the filter cake and remove the trapped impurities from the tubes which then flow through a sludge opening in the bottom of the filter chamber to a sludge receiver. Backflushing or backwashing can be accomplished by introducing compressed air into the filtrate chamber of the system. The filter tubes are then recoated with fresh clean diatomaceous earth to reform the filter cake prior to resuming normal operation of the filter system.
The problem with the prior art's manner of regenerating the filter cake once it has been clogged with solids removed from the filtrate is that it uses the filter powder or material only once. Further, the cleaning of the filter system and precoating of the filter tubes requires time during which the filter system is out of operation or production and thus reduces the overall capacity of the filter system.
There have been attempts in the past to reuse and regenerate the filter cake without dumping the filter media each time. These procedures, up until now, have proven to be only partially successful and produce additional problems not encountered before. When the pressure drop across the filter tubes increases to a point that indicates that the surface of the filter cake is substantially clogged with contaminants or sludge, a vibrator or hammering device has been applied to the tube support sheet in an effort to dislodge and break up the filter cake from the tubes. Theoretically, the filter cake is to be placed back into solution in the liquid, but the hammering only partially breaks the cake loose from the tubes and what cake is removed usually remains in relatively large chunks preventing an even regeneration of the cake on the tubes. In addition, another problem occurs in that the mechanical forces on the tube sheet and vessel can cause leakage around the tube flanges or even in the vessel flanges. An externally leaking filter is naturally undesirable. An internally leaking filter is intolerable in that the contaminants pass through to the outlet defeating the purpose or usefulness of the filter.